This invention is in the field of stents that are used to maintain patency of a vessel of the human body.
It has been shown that intravascular stents are an excellent means to maintain the patency of blood vessels following balloon angioplasty. Robert, David and Tim Fischells in U.S. Pat. No. 5,695,516 describe a stent design which elongates as it begins expansion. The Fischells in U.S. Pat. No. 5,697,971 describe a stent which also elongates as it begins to expand and includes xe2x80x9cSxe2x80x9d shaped undulating longitudinal structures to enhance flexibility and side branch access. Both these designs lengthen at first then shorten as they expand to larger diameters. Beyond a certain point, these designs can actually shorten more than designs which do not initially lengthen.
This invention is an improvement of the stent design described in U.S. Pat. No. 5,697,971 to reduce shortening during stent expansion. The present invention is a balloon expandable stent made from either fully annealed stainless steel or a metal of equivalent xe2x80x9csoftnessxe2x80x9d. This stent also has expandable circumferential sets of strut members connected by undulating longitudinal structures such as those of the prior art. The present invention is different in that the undulating longitudinals are specifically adapted to stretch during balloon expansion so as to reduce stent shortening.
Because friction exists between the balloon and the stent""s inside surface as the stent is expanded by the balloon, each circumferential set of strut members will tend to maintain its longitudinal position on the balloon. If there were no interconnections between the circumferential sets of strut members, the average distance between the circumferential sets of strut members would stay the same and each set of strut members would decrease in its longitudinal dimension. The relatively stiff undulating longitudinal structures of the prior art which resist stretching will pull the circumferential sets of strut members toward each other as the stent expands causing the stent to shorten. If however, the undulating longitudinal structures are weak and easily stretched by forces less than the friction between each circumferential set of strut members and the balloon, then the undulating longitudinal structures will be permanently elongated. If as the stent expands, the circumferential sets of strut members maintain their longitudinal position, and the undulating longitudinal structures stretch to compensate for the longitudinal shortening of each circumferential set of strut members, then the stent will barely shorten.
Thus it is an object of this invention to have a stent with reduced shortening as a result of the permanent longitudinal stretching of undulating longitudinal structures as the stent is radially expanded.
Another object of this invention is to have the frictional forces between adjacent circumferential sets of strut members and the balloon being sufficiently great during balloon inflation so that the metal of the undulating longitudinal structures that connect adjacent sets of strut members is stressed beyond its elastic limit.
Still another object of this invention is to have the width of the undulating longitudinal structures be significantly less than the width of other stent struts.